This invention relates to pressure-sensitive adhesive tape and, more particularly, to polyurethane adhesive tape which is both clear and thermally stable or flame retardant.
Pressure sensitive adhesive tapes, including both roll tape and sheet forms, have long been prepared both as general purpose tapes and as specialty tapes. For applications where optical clarity or adhesive bond longevity is required, acrylic or silicone pressure-sensitive adhesives have traditionally been used. In certain applications, however, such traditional materials do not meet the performance and cost requirements. In particular, for example, various aeronautical applications are found to require pressure-sensitive adhesive tape which meet requirements for low flamability. More specifically, certain applications, especially certain aeronautical applications call for pressure-sensitive adhesive tape having maximum average burn extinguishing time of 15 seconds, maximum average burn length of 8 inches and maximum average drip extinguishing time of 5 seconds, measured in accordance with BSS 7230 Method F2 (12 seconds vertical) published by Boeing Company, (Seattle, Wash., U.S.A.) and set forth in substantial portion as test method 6, below.
Existing tapes meeting such stringent requirements have been found to have shortcomings in other respects. Most notably, it has now been recognized that pressure-sensitive adhesive tape meeting the above-stated thermal stability requirements would and other physical property requirements would also advantageously be substantially clear or transparent. In addition, in certain applications the substantially clear, thermally stable tape must also meet physical property requirements such as weight, elongation, tensile strength, peel,strength, water vapor impermeability, environmental resistance, moisture absorption and puncture resistance. A self-adhesive tape is described in U.S. Pat. No. 5,807,637, which is said to be capable of gap-bridging sealing functions and to form bonds which are stable at both high and low temperatures. Specifically, a double-sided tape is described, based on a cross-linked, non-foam polyurethane substrate. The adhesive compositions suggested for this double-sided tape are known adhesive compositions based on rubber or acrylate chemistry. A high quantity of fillers are employed, however, which are said to contribute to the permanent bond strength achieved by the double sided tape. Fillers such as carbon black, chalk, zinc oxide and the like are disclosed which would render the tape non-transparent. In addition, the cross-linked polyurethane employed in U.S. Pat. No. 5,807,637 would be expected to render the tape undesirably brittle. Adhesive tape employing a polyurethane acrylate layer is described in U.S. Pat. No. 5,858,495 to have a high degree of three-dimensional cross-linking, with resulting brittleness sufficient to prevent removal of the tape from a substrate except in small segments. The pressure sensitive adhesive tape of U.S. Pat. No. 5,858,495 also employs pigment, such that it would not be substantially transparent.
It is an object of the present invention to provide substantially clear adhesive tape which is thermally stable, that is, meeting the above-stated specifications for flame retardancy. It is an object of at least preferred embodiments of the invention to also meet certain physical property requirements. These and other objects of the present invention will be better understood from the following disclosure of the invention and detailed description of certain preferred embodiments.
In accordance with one aspect, the present invention provides thermally stable, clear, pressure-sensitive adhesive tape comprising a clear polyether-based polyurethane backing film and clear, polyether-based polyurethane pressure-sensitive adhesive coated on one side of the backing film. The polyurethane backing film and/or the polyurethane adhesive incorporates flame retardant effective to render the tape substantially flame retardant. As used here and in the appended claims, the term xe2x80x9cflame retardantxe2x80x9d means that the pressure-sensitive adhesive tape disclosed here meets the above-stated requirements for thermal stability, specifically, having maximum average burn extinguishing time of 15 seconds, maximum average burn length of 8 inches and maximum average drip extinguishing time of 5 seconds, measured in accordance with BSS 7230 Method F2 (12 seconds vertical) published by Boeing Company and set forth in substantial part as Test Method 6, below. Preferably, both the polyurethane backing film and the solvent based polyurethane incorporate a flame retardant additive.
The term xe2x80x9cclearxe2x80x9d or xe2x80x9csubstantially clearxe2x80x9d as used here and in the appended claims, means that the PSA tape disclosed here, as applied to the substrate, is substantially transparent, preferably being sufficiently transparent for a person applying the tape to a substrate to view the perimeter of any air bubbles trapped between the tape and the substrate, to which it has been applied, and to view a gap or seam in the underlying substrate through the applied tape and read 12 point OCRA Extended typeface printing. Although the PSA tape disclosed here most preferably is colorless, it optionally may be tinted with color and, nevertheless, be substantially transparent, that is, xe2x80x9cclearxe2x80x9d as that term is used here.
It will be recognized by those skilled in the art, i.e., those skilled in the field of pressure-sensitive adhesives tapes, that the present invention represents a useful technological advance. More specifically, as further discussed below in connection with certain preferred embodiments, the clear polyurethane backing film and clear, pressure sensitive polyurethane adhesive coated on one side of the polyurethane backing film provides performance characteristics meeting the requirements of certain end use applications, including certain aeronautical applications. In particular, certain preferred embodiments of the adhesive tapes disclosed here provide advantageous performance both in being applied to a substrate and in subsequent performance. More particularly, as noted above, the pressure-sensitive adhesive tapes disclosed here meet the thermal stability requirements applicable to certain aeronautical applications, as well as uses such as floor panel joint protection, galley area floor membrane and lead edge protection. Moreover, it is a particular advantage of the adhesive tapes disclosed here, that they are clear, that is, substantially transparent. This enables air bubbles trapped under the tape to be viewed during application of the tape, so that the air bubbles can be removed to provide better adhesive bonding of the tape to the substrate. In addition, it is possible to view through the tape to observe the location of a seam or gap or the like being sealed by application of the tape, such that the tape can be better centered or otherwise positioned along the seam as it is applied. In addition, it is possible to view through the tape to observe alignment marks or part identification numbers as an additional safeguard to proper assembly of the structure. Additional features and advantages of the invention will be better understood from the following detailed description of certain preferred embodiments.
The preferred embodiments described here in detail have a clear polyether-based polyurethane backing film. Most preferably, the backing film is a mono-layer backing, that is, a backing formed as a single layer of clear, substantially non-crosslinked polyether-based polyurethane rather than being constructed as a laminate of multiple layers. Suitable materials for the backing film include, for example, polyurethane X-1656 from Deerfield Urethane, Inc. Whately, Mass. and Stevens Urethane, Holyoke, Mass. Other suitable polyether-based polyurethanes for the clear backing film of the PSA tapes disclosed here are commercially available or can be prepared in accordance with known materials, equipment and processes, and will be apparent to those skilled in the art, given the benefit of this disclosure.
As disclosed above, the thermally stable, clear adhesive tape disclosed here incorporates flame retardant. Flame retardant incorporated into the clear polyurethane backing film material must be compatible with the polyurethane material of the backing film and, in particular, must not interfere with the substantial transparency and other performance characteristics of the backing film. Exemplary flame retardants suitable for use in the polyurethane backing film materials disclosed above include FIRE MASTER(trademark) HP-36 available from Great Lakes Chemical Corporation, Wilmington, Del., and other commercially available flame retardants such as Reotos 3600 and Kronitex TBP. Additional commercially available flame retardants and other materials suitable for use in the clear polyurethane adhesive tapes disclosed here will be apparent to those skilled in the art in view of the present disclosure. The flame retardant is employed in an amount sufficient, acting together with flame retardant incorporated in the polyurethane adhesive discussed further below, such that the clear adhesive tape meets the requirements of having maximum average burn extinguishing time of 15 seconds, maximum average burn length of 8 inches and maximum average drip extinguishing time of 5 seconds, measured in accordance with BSS 7230 Method F2 (12 seconds vertical) published by Boeing Aircraft Company. The backing is preferably polyether polyurethane having ASTM D-2240 Shore A hardness of at least 85, ASTM D-882 100% modulus of at least 900 psi, ASTM D-1525 Vicat softening temperature of at least 165xc2x0 F., ASTM D-470 split tear strength of at least 125 lb/inch, glass transition temperature between xe2x88x9265xc2x0 F. and xe2x88x9225xc2x0 F., and ASTM D-1044 Taber abrasion of between 2 and 10 mg/1000 cycles.
Optionally, additional additives may be employed in the polyether-based polyurethane material of the backing film. All such additives must be compatible with the polyurethane and must not interfere with the substantial transparency, flame retarding properties or other performance characteristics of the tape. Exemplary additives include colorants, antiblocking agents and processing lubricants. Additional additives suitable for use in the clear polyurethane adhesive tapes disclosed here will be apparent to those skilled in the art in view of the present disclosure.
The thermally stable, clear adhesive tape further comprises clear, pressure-sensitive, solvent-based polyether-based polyurethane adhesive coated on one side of the backing film. The pressure-sensitive polyurethane adhesive preferably is applied as a single layer, that is, a mono-layer, on a mono-layer polyurethane backing film, rather than being constructed as multiple layers of different adhesives, both to provide simplicity and cost-advantage in the manufacture of the tape and to maintain good transparency. The polyurethane adhesive preferably is of polyether polyurethane type with specific gravity between 1.0 and 1.25 and is readily solvated for liquid coating. Suitable solvents for use in the pressure sensitive polyurethane adhesive include, for example, those which readily dissolve the pressure sensitive polyurethane adhesive and are readily driven off in the range of drying temperatures between 175xc2x0 F. and 325xc2x0 F. Exemplary solvents suitable for use in the clear adhesive tapes disclosed here include toluene, isopropyl alcohol and ethyl acetate. The amount of solvent used preferably is 30-70% by weight. Additional and alternative solvents suitable for the clear, flame regardant PSA types of the present invention will be apparent to those skilled in the art in view of the present disclosure.
Flame retardant incorporated into the clear, polyether-based polyurethane adhesive must be compatible with the polyurethane material of the adhesive and with the other components of the PSA tape. In particular, it must not interfere with the substantial transparency and performance characteristics of the adhesive tape. Exemplary flame retardants suitable for use in the polyurethane adhesive include those described above. Additional commercially available flame retardants and other flame retardant materials suitable for use in the polyurethane adhesive of the clear, flame retardant PSA adhesive tapes disclosed here will be apparent to those skilled in the art given the benefit of the present disclosure.
Optionally, additional additives may be employed in the polyurethane adhesive. All such additives must be compatible with the polyurethane adhesive and the other components of the PSA tape, and must not interfere with the substantial transparency, flame retarding properties or other performance characteristics of the tape. Exemplary additives include tackifiers, reinforcing agents and colorants. Additional optional additives will be apparent to those skilled in the art given the benefit of the present disclosure.
The polyurethane adhesive can be applied to the polyurethane backing film, preferably as a mono-layer as indicated above, by any suitable method. Exemplary methods include, for example, reverse roll coating, knife over roll coating and slot die coating. The thickness of the adhesive on the backing film typically will be about 0.5 mil to 10 mil, more preferably about 1 mil to 4 mil, for example, about 2.5 mil. The thickness of the adhesive layer will vary with any particular tape of the present invention, depending, in part, on the particular end use application for which it is intended. In accordance with certain preferred manufacturing techniques, elongate, wide sheets of the polyether-based polyurethane backing film is are coated with the polyether-based polyurethane adhesive, after which the adhesive coated backing film is cut longitudinally to desired tape widths and then rolled, optionally, with a release liner.
As used here and in the appended claims, reference to the pressure-sensitive adhesives tapes disclosed here as being xe2x80x9cclearxe2x80x9d should be understood to mean that the adhesive tape will be substantially clear as applied to the intended substrate. Even prior to application, however, the tape is sufficiently clear to permit viewing of a seam line or the like as it is being applied, notwithstanding a possible minor frosted appearance or the like. The combination of solvent-based polyether-based polyurethane adhesive coated on the clear polyether-based polyurethane backing film discussed above is found to provide excellent performance characteristics, especially well-suited to aeronautical applications such as seam sealing. In particular, this novel combination of materials, in addition to providing the advantages of a clear PSA tape, is found to have excellent performance characteristics. Certain especially preferred embodiments, in addition to being clear and having the requisite flame retardancy, are found to have the following physical properties and performance characteristics:
These especially preferred embodiments, as recited in the table above, have elongation to break of at least 600% and tensile strength of 6,500 psi. Certain embodiments of the clear tape disclosed here achieve at least 675% elongation, and in some embodiments even achieve 775% elongation. Also, certain embodiments achieve at least 7250 lbs/sq. inch and in some embodiments even achieve 8100 lbs/sq. inch. Also, certain embodiments achieve at least 100 pounds puncture test values. The test methods 1 through 8 referred to in Table A above are set forth below in the section under the heading EXAMPLES.